Celebi
Jirachi
Shaymin
Victini
So... The ONE Mew-equivalent has always been there.
But there are tons of base 600 event legendaries which are not the standard Pixie.
Manaphy, Darkrai, Deoxys, Genesect, a bunch of others from BW that I don't remember...
Instead of making tons of legendaries they could make quests or events relating to previous legendaries, there are enough of them. Most of the new ones are terribly boring and the huge amount of legendaries introduced in DP and BW was totally unnecessary. Most of them are overdesigned anyway. I'm not one of those who think the new generations are worse design-wise (they're not), but the legendaries definitely are. Arceus is the moment the "legendary" concept officially jumped the shark. If there's a GOD pokemon the very least it could do given its godly powers is to make itself not as ugly as it is.
I'd like them to start closing loose evolutionary ends and make more Pokemon viable. Off the top of my head:
Farfetch'd
Pinsir
Mr Mime
Jynx
Dunsparce
Luvdisc
Mantine
Delibird
Solrock and the other guy
Zangoose (definitely)
Absol (definitely)
Seviper
I really liked how DP was full of new evolutions to old pokemon. There's a lot of potential there. They are already rehashing stuff like fire monkeys and fire foxes so why not go and make the most of pokemon that they already created.